In recent pathological examinations, an examiner observes pathological specimens (samples) not only directly with his/her eyes through an optical microscope, but also through image data taken by a virtual microscope and displayed on a display device. Such a virtual microscope enables plural examiners to simultaneously observe the image data of the pathological specimen displayed on the display device. Moreover, the virtual microscope provides many advantages including sharing of the image data with a distant pathologist to ask him/her for diagnosis. However, this method causes a problem of taking time because the image data is prepared by image taking of the pathology specimen.
One reason for taking time is that the image data is taken by using a conventional virtual microscope with a narrow image taking area with respect to a large pathological specimen. Such a narrow image taking area of the conventional virtual microscope makes taking of images of plural separate areas in the large pathological specimen, in other words, scanning of the pathological specimen, to combine the taken images into one image necessary. Therefore, an optical system (image-taking optical system) having a large image taking area is requested for the virtual microscope in order to reduce the number of times of image taking of the pathological specimen to shorten the time to obtain the image data as compared with the conventional virtual microscope.
Moreover, an optical system telecentric on an object side and an image side (that is, both sides) is requested in order to decrease influence (magnification variation or the like) on the image data due to positional errors of the pathological specimen and an image pickup element such as a CCD sensor in an image-taking optical axis direction.
In addition, for detailed observation of the pathological specimen, an optical system with not only the above-mentioned large image taking area but also a high resolving power in a visible wavelength range. Such an optical system having a high resolving power is requested for not only the pathological examinations but also various fields.
Japanese Patent Laid-Open No. 60-034737 discloses a microscope objective lens formed as a dioptric system whose aberrations are sufficiently reduced over the entire visible wavelength range and suitable for observing biological cells or the like. However, although the microscope objective lens disclosed in Japanese Patent Laid-Open No. 60-034737 sufficiently reduces the aberrations over the entire visible wavelength range, it has an observation area which is not necessarily sufficiently large.
Furthermore, Japanese translation of PCT international application No. 2001-517806 discloses a super-wideband ultraviolet microscope video system using a catadioptric system to achieve a high resolving power over a wide ultraviolet band, which is used for inspecting defects existing in integrated circuits and photomasks. However, although the super-wideband ultraviolet microscope video system disclosed in Japanese translation of PCT international application No. 2001-517806 achieves a high resolving power by sufficiently reducing aberrations over the wide ultraviolet band, it has a field-of-view which is not necessarily sufficiently large.
Moreover, Japanese Patent Laid-Open No. 2002-082285 discloses a catadioptric system of an exposure apparatus suitable for exposing minute patterns on a wide area to manufacture semiconductor devices. However, although the catadioptric system disclosed in Japanese Patent Laid-Open No. 2002-082285 achieves a high resolving power by sufficiently reducing aberrations over the wide area, it has a long total length. Such a long total optical system length increases the size of the exposure apparatus, which is inconvenient to install and operate the apparatus.